The invention relates to a method for picture insertion into video pictures, as is employed for example with television pictures. In the case of picture-in-picture insertion, a smaller second picture is inset into the usual television picture and can be viewed at the same time as the television picture. Picture insertion also includes the display of subtitles, operating menus and other representations which are generated by a character generator for display in the main picture.
For picture-in-picture insertion in video signal processing apparatuses, in particular television receivers, it is necessary to buffer-store the picture to be inserted in a frame memory in order to delay the picture signal of the picture to be inserted, so that it can be displayed synchronously with the main picture at a predetermined location on a display device, for example a screen. Synchronization with the main picture is effected using vertical and horizontal synchronizing signals in this case. In particular disturbances in the horizontal pulses become apparent in the horizontal picture position by virtue of shaking and displacements.
For synchronization of the insertion channel for the picture to be inserted with the main channel of the main picture, circuit arrangements with phase shifters are suitable, in which an output clock signal coupled to a reference pulse is generated, said signal having a high phase coupling.
In a phase shifter, the clock frequency is fixed. The number of clock cycles between two horizontal synchronizing signals of the main channel is dependent on the time interval between said signals. Synchronization is produced when the edge of the horizontal signal temporally coincides with the edge of the clock signal for the insertion channel.
A circuit arrangement for generating an output clock signal coupled to a reference pulse is described in DE 195 06 543 C1.
Line deflection in television receivers takes place independently of the picture signals. Therefore, signals of the main channel with a different line duration are always imaged onto the same section of the screen. Since the signals of the insertion channel are coupled to those of the main channel, changes in the line duration lead to position displacements of the inserted picture.
An example will illustrate this. A signal of the main channel with a length of 1 μs is imaged onto 1 cm of the screen. If the line duration changes by 5%, i.e. the signal is now 1.05 μs long, 1 cm can still be seen. However, a signal inserted after 10 μs is inserted after 10 cm, and after 9.5 cm in the case of the changed line duration, because the clock of the phase shifter is independent of the line duration of the main channel.
Since, in the case of a fluctuating line duration, the picture to be inserted changes its position within the main picture a number of times per second, a viewer is given the impression of a jittery picture to be inserted.